Cermets are that class of materials being a heterogeneous mixture of ceramic and metal phases or particles finely interspersed or intimately intermixed with each other. Cf. U.S. Pat. No. 3,421,863, column 1, lines 29-35, and Ceramic Glossary published 1963 by The American Ceramic Society, page 9. More commonly, cermets are formed by physically combining separately pre-existing ceramic and metal materials. However, as earlier disclosed in U.S. Pat. No. 2,702,750, some cermets may be produced with in situ formation of a metal phase in a solid mass of silica-containing ceramic material by contacting that solid mass with molten metal (e.g. aluminum or alloys thereof) at about 700.degree. to 900.degree.C. such that the contacted mass remains substantially solid and retains its shape and size. Upon such contact, the molten metal reacts with the solid mass by reducing the silica therein to silicon and replacing it with simultaneously formed crystalline oxide of the contacting metal (e.g. alumina) and with some of the contacting metal itself, both oxide and metal being thoroughly and intimately interspersed in the transformed crystalline solid mass. The resultant silicon is mainly found dissolved in the excess contacting metal which is external to the transformed solid mass. This latter type of cermet is sometimes designated as a "reaction cermet". The suggested uses of such cermets include electrical conductors and/or resistance elements, refractory articles and abrasive articles. For the latter articles, it is proposed to transform silica sand into cermet abrasive grains of alumina and aluminum-base metal, which may be ceramically bonded to form grinding wheels (similar to those of the common fused or sintered alumina abrasive grains). Alternatively, it is proposed to transform a fused silica disk into a cermet grinding wheel of alumina and aluminum-base metal.
While acknowledging reaction cermets to be "characterized by a highly refractory nature, a high degree of hardness, good electrical conductivity and variable abrasive characteristics," U.S. Pat. No. 3,034,908 discloses employing an alloy of aluminum with 20-35 wt.% silicon at 900.degree. to 950.degree.C. in the process of making a reaction cermet body from a silicate glass body so that, unlike the products of earlier processes, such cermet body will have a more homogeneous composition and structure with no apparent defects (such as voids or cracks).
A variant reaction cermet is described in U.S. Pat. No. 3,295,934 as having a matrix which is rich in aluminum and in which there is a dispersed phase of granular glass particles with a surface layer of alumina resulting from limited reaction of the glass and aluminum during heating of a pressed preform at about 600.degree. to 800.degree.C. for 5 to 40 minutes. The preform is made from a mixture of 25-50 wt.% moderately reactive glass particles and 75-50 wt.% aluminum-rich metal particles. A moderately reactive glass may contain 45-57 wt.% SiO.sub.2, 15-28 wt.% MgO, and 20-30 wt.% Al.sub.2 O.sub.3 wherein SiO.sub.2 + MgO + Al.sub.2 O.sub.3 is at least 80 wt.%. This type of cermet is said to have great hardness and high resistance to shearing and crushing. It is recommended for cylinder heads and pistons of internal combustion engines and other items subjected to temperatures up to 400.degree.C. However, tests made on this type of cermet showed that, in addition to the large amount of metal phase therein, it had average crystal sizes on the order of twenty to forty microns and abrasive grit size particles exhibited high attritious wear rate, e.g. about 30 .times. 10.sup.-.sup.6 inch/cut.
More recently, it was proposed in U.S. Pat. No. 3,437,468 to make abrasive grain of reaction cermet type of composite material by covering a molten pool of aluminum at a temperature of at least 1000.degree.C. with a layer of finely divided magnesium silicate. After reaction with the molten aluminum, the transformed silicate is a composite material composed of (by weight) 35 to 55% magnesium aluminate spinel, 15 to 35% alpha alumina and 20 to 40% of an elemental phase comprising free silicon and/or a silicon-aluminum intermetallic with optional presence of free aluminum. The prominence of silicon in the elemental or metal phase in such reaction cermet is apparently due, at least in part, to the higher reaction temperature vis a vis the earlier reaction cermets described above.
Despite the foregoing developments, reaction cermets have not found, to my knowledge, any apparent commercial and industrial favor or usage as abrasive grain.
A continuing major focus in the abrasives industry today is the development of more efficient abrasive grain having longer lifespan for heavy duty snag grinding (also called "snagging") of ingots and castings of metal (e.g. steels).
As known to me, the presently best abrasive grains (i.e. particulate abrasive material) that are commercially available and employed for heavy duty snag grinding consist of a fused mixture of alumina and zirconia, both of high purity quality. The two most common compositions of these commercial abrasives appear to have approximate Al.sub.2 O.sub.3 :ZrO.sub.2 weight ratios of 3:2 and 3:1. The development of these fused alumina-zirconia grains substantially overcame such deficiencies in prior abrasive grains as the relatively high rates of chipping due to brittleness of widely used fused grains and the relatively high rates of attritious wear due to abrasive softness of zirconia grains and sintered alumina grains. Both deficiencies contribute to low efficiency, which is usually the ratio of metal removal to grinding wheel wear. However, there still remains a strong desire of users of the abrasive grains for even further improvments that will afford them significant economic advantage.